Generally, embankments are installed in rivers to block a river and maintain the amount and level of water in an upstream side constant and are provided with a water gate to allow a desired amount of water to flow to the downstream side. Thus, the embankments can prevent a variety of damage caused by water, for example, flooding, and make it possible for blocked water to be used as water for agricultural or industrial use or for living.
However, such an embankment is installed across a river to block the flow of water, thereby impeding upstream or downstream movement of various kinds of fish that depend on a variation in their seasonal inhabited environment. Thus, the aquatic ecosystems between the upstream side and the downstream of the river are disconnected from each other. Particularly, leptocephali, which are the larvae of eels, are an endangered species that move in groups to the upstream side of a river along the bottom of the river in the spring. If there is an embankment in the river, it blocks the movement of leptocephali. Due to this, leptocephali are easily eaten by predators. Consequently, there is a problem of a rapid reduction in the population of eels.
Moreover, embankments impede the flow of lower-layer water that is disposed around the bottom of a river, whereby river water may become excessively contaminated, or algae may be excessively created. Consequently, water quality deteriorates and, particularly, contamination of the bottom of the river is increased. As such, existing embankments have a problem in that they cause destruction of the environment.
In addition, if a separate water gate is installed in an embankment, it may distort the normal flow of a river because water gates are typically disposed at positions at which maintenance and management thereof can be easily conducted. Thereby, a river-bed scouring phenomenon is caused, and a danger of collapse of the embankment is increased. Furthermore, the performance of flood control is limited, so there are problems in maintenance and management.
Of course, various styles of fishways through which fish can move may be formed in embankments. For example, a safety fishway that makes it possible for fish to safely move or flee was proposed in Korean Patent Registration No. 10-1046036. This safety fishway includes: an intake and exhaust valve 7; a horizontal pipe 2 provided with a transparent window 6; inclined pipes 3 that extend downward from the horizontal pipe 2 at a predetermined angle relative to the horizontal pipe 2 ; extension pipes 4 that horizontally extend the respective inclined pipes 3 ; entering-and-exiting pipes 5 that extend upward from the respective extension pipes 4; and covers 8 that cover the respective entering-and-exiting pipes 5.
However, in the above-described conventional safety fishway 1, the performance thereof is limited depending on a difference in height between the inclined pipes disposed on opposite sides of the embankment D, a difference in water level, and an angle of inclination of each inclined pipe. Thus, it may be difficult for fish to easily move through the fishway 1. Furthermore, the conventional safety fishway 1 is open, so that not only fry or small fish that are weak in swimming force but also large carnivorous fish can easily enter the safety fishway 1. Therefore, the conventional safety fishway 1 cannot provide a safe shelter for fry or small fish.
Furthermore, the conventional safety fishway 1 may further include a water gate that discharges stored water to control the level or quantity of water. However, only upper-layer water that has been newly drawn in is discharged. Thus, the bottom of a river is gradually contaminated. Because water cannot circulate vertically, algae may be excessively created, thus causing a red tide phenomenon, thereby further deteriorating the water quality environment.
Moreover, the conventional safety fishway 1 is hazardous to leptocephali, which are an endangered species all over the world, in that they cannot move to the upstream side and they may be swept away in strong water currents or eaten by predators entering the fishway. Therefore, the conventional safety fishway 1 cannot solve a fundamental problem of a rapid reduction in the population of eels.